Many vehicles have one or more tires that may deflate or go completely flat during travel without the driver being immediately aware of the condition. These include tractor-trailer combinations, motor homes, buses and various types of vehicles and trailers being towed that have axles mounted with two or more tires, or tandem rear axles that may have a total of four, six or eight tires. Unlike those of motorcars, drivers of these vehicles cannot sense that a tire has low pressure or suffered a blowout. The large size and heavy weight of these vehicles isolate a driver from noise and vibration created by a blowout. Likewise, a driver towing another vehicle is insulated from any sensation of a deflating or flat tire on the towed vehicle.
Low tire pressure can often lead to a blowout of the tire if it goes undetected. While traveling on an interstate highway at speeds of between 55 and 65 m.p.h., a blowout can cause extensive damage, especially when it goes undetected. The tire disintegrates and its debris damages the vehicle or trailer, often extensively. The wheel hits against the pavement, and usually suffers damage. In the case of dual rear wheels, where there is four wheels per axle, continued motion with one of the tires deflated may even cause the deflated tire to catch fire. There have been cases reported in which such a fire has caused total loss of the vehicle or trailer. Most disconcerting about a loss of pressure or low pressure in a vehicle's tire is, however, that it affects the load carrying capability, steering, breaking and overall control of the vehicle. Loss of a load bearing tire may result in a driver losing control and causing damage to the vehicle, passengers and other property and passengers of other vehicles.
Needless to say, the continual prospect of suffering a blown tire makes drivers of buses, motorhomes, tractors and other large or towing vehicles very uneasy.
Because a tire with abnormally low pressure is a candidate for a blowout, a system which monitors all the tires and provides early warning of an abnormally low pressure would go a long way toward providing peace of mind to the drivers, as well as provide improved safety, prolong the life of tires, prevent tire destruction and fires, and save time. However, though instrument panels in vehicles are over-populated by gauges and lights for providing all sorts of warning, most vehicles do not have a tire pressure monitor that drivers who spend a lot of time on the road desperately want and need. In addition to the failure to recognize the importance of detecting at an early stage low tire pressure to prevent blowouts, there is at least one other reason for the absence of tire pressure monitoring systems: prior art tire pressure monitors have adopted expensive and impractical approaches to this problem.
In the art, there are several examples of tire pressure monitors and alarm systems. These are typically fastened to the rim of the wheel and require that a hole be drilled through the wheel. See U.S. Pat. No. 4,954,677 of Alberter et al.; U.S. Pat. No. 4,894,639 of Schmierer; U.S. Pat. No. 4,866,982 of Gault; U.S. Pat. No. 4,768,375 of Echardt et al.; and U.S. Pat. No. 4,784,993 of Lothar et al. These systems include a transducer of some sort that converts the pressure to a signal for communicating the pressure to a remote display.
The disadvantage of these tire pressure monitors is that the transducers are mounted through the wheel rim. Thus, the wheel must be either specially manufactured or adapted (if possible) with holes that are drilled in the wheel to receive the transducers. As holes cause undue stress on the wheel retrofitting preexisting wheels, it gives rise to safety and liability problems. Thus, they must be manufactured for these systems as original equipment. However, they must meet strict Department of Transportation guidelines and undergo stress tests before approval.
These systems also require that the wheel be removed from the vehicle and disassembled to gain access to the transducers for service. Furthermore, they require some sort of electrical connection between the transducer and any remote monitoring device. With a rotating wheel, this electrical connection requires special contacts, complicating the system, introducing added cost and reducing reliability.
The problem of connecting the transducer to a monitor has been solved in part by radio frequency communications. As shown in U.S. Pat. No. 4,890,090 of Ballyns, a pressure transducer is coupled to a radio frequency transmitter that is mounted within the tire and secured to the wheel rim. Although it has the advantage of wireless communication of the pressure to a remotely placed monitor, it suffers from the same disadvantages of the rim mounted transducers: it is difficult to install and service, and requires special adaption of the wheel.
To avoid this communication problem, it is possible to indirectly monitor the condition of the tire using tire rotation sensors like those installed as original equipment on vehicles with anti-lock braking and some all-wheel drive systems. To detect a deflating tire, these sensors are monitored for abnormal changes in rotation speeds of the tire indicating deflation. Doing so requires sophisticated sensors, data processing equipment and algorithms, and a vehicle originally equipped with this advanced and expensive technology. It is a sophisticated approach, but one that is not feasible for most vehicles such as buses, trucks and motor homes currently being manufactured and on the road that are not using this technology.
Another approach avoids altogether mounting transducers on a tire. Yet it is just as complicated and expensive. An elaborate, and extremely expensive air pressure line is built into the car that runs from a wheel, through a hub and down an axle to a sensor located within the vehicle. This approach is generally available only to the most sophisticated and expensive vehicles and must be installed as original equipment.
Despite previous substantial efforts to improve the safety of tires, current tire pressure monitoring systems continue to run in the vein of being expensive and elaborate; they require substantial modification to wheels and to the car for their use; and they offer methods having little to no feasibility for retrofitting the millions of ordinary wheels that are in use and will continue to be manufactured and used.